The present invention is directed to microporous poly(tetrafluoroethylene)/elastomer vascular grafts which have intrinsic elasticity, and methods of preparing the same. Specifically, the present invention is directed to an expanded porous, poly(tetrafluoroethylene)/elastomer vascular graft formed by the respective curing and sintering of the elastomer and poly(tetrafluoroethylene) in a two step manner to impart intrinsic elasticity to the elastomer portion of the graft.
Co-pending application Ser. No. 892,271, now abandoned, entitled "POROUS HIGHLY EXPANDED FLUOROPOLYMERS PROCESS THEREFOR", incorporated herein by reference, discloses the use of elastomers for strengthening expanded poly(tetrafluoroethylene), also known as PTFE, articles. It is believed that the elastomer coats the PTFE fibrils, and forms a continuous matrix interpenetrating the microstructure of the fibrils. In so doing, it renders the poly(tetrafluoroethylene) structure porous, yet durable with excellent pliability for use as a vascular graft. More importantly, the addition of an elastomer to the poly(tetrafluoroethylene) provides a material from which vascular grafts can be made which is biological compatible with the patient's tissue surrounding the graft after implantation.
While the process disclosed in U.S.S.N. No. 892,271 can be used to formulate poly(tetrafluoroethylene)-elastomer articles, the present invention relates to a process and product where there is an improvement in the compliance, elasticity, flexibility and strength of the poly(tetrafluoroethylene)-elastomer article due to a novel expansion, and sequential curing and sintering of the elastomer and PTFE. The present invention relates to rods, tubes, sheets, or any products produced thereby, but more particularly relates to shaped articles which are useful as medical implants. The invention, as described herein, emphasizes the process as directed to produce medical implants or specifically, vascular grafts, which have the improved physical characteristics, but should not be limited to exclude the application of the process for other products.
It is generally difficult to manufacture a highly porous poly(tetrafluoroethylene) material by the expansion of the PTFE at a very high ratio in a single expansion step. The fibrils, which are fragile, may not develop completely before being torn during the expansion process. Those processes using PTFE require that the PTFE article be stretched multiple times. These processes provide for an expanded PTFE article, and particularly vascular grafts having the desired microporosity not achieved by the use of PTFE alone.
Examples of processes which perform multiple expansion of PTFE articles, including vascular grafts, are disclosed in U.S. Pat. Nos. 2,586,357, 3,953,566, 3,962,153, 4,110,392, 4,248,924, and Japanese patent publication No. 13560/1967. Japanese patent number 13560/1967, discloses a process of producing porous articles of PTFE by the paste forming extrusion of PTFE product which is stretched more than once. The stretched article, as exemplified a sheet, is then sintered. This reference further teaches the impregnation of the formed PTFE sheet with an elastomer, which elastomer is cured. The Pat. No. 3,962,153 describes a conventional process for stretching poly(tetrafluoroethylene) materials at a rate exceeding 2000% per second to a final length of up to 1760 times the original length of the material. Another multiple stretching process is disclosed in U.S. Pat. No. 4,110,392, wherein a PTFE article is first stretched, while in the unsintered state, then free sintered, followed by a second stretching.
Conventional vascular grafts manufactured from poly(tetrafluoroethylene), which has been expanded to provide for the microporosity, often possess limitations in strength and compliance. The expanded, porous grafts do not hold or resist dilation unless wrapped with a re-enforcing film or fiber for support. This is because of the relatively low radial tensile strength of poly(tetrafluoroethylene). The reinforcement slows down the tissue ingrowth preventing rapid healing. In addition, the grafts are stiff and non-compliant in relation to a natural artery. A porous flexible structure that is closer in compliance to a natural vessel will help prevent the complications resulting from the aforementioned detrimental characteristics.
As stated, the incorporation of an elastomer into the PTFE matrix, by the blending of the PTFE and elastomer, and then forming the desired article, as taught by the Co-pending application Ser. No. 892,271, provides a strengthened PTFE product. Another reference which teaches the general incorporation of a specific elastomer, a fluoroelastomer, is disclosed in U.S. Pat. No. 4,555,543.
While these later references teach the use of an elastomer, which is blended into the poly(tetrafluoroethylene) resin, the resulting article is still manufactured by the multiple stretching techniques discussed in the above patent references. These techniques provide that the article is stretched numerous times, with the final article being sintered. While the resulting product possess some elastic memory due to the presence of the elastomer, it would be even more advantageous to provide the article with an elastomeric component which would inherently resist the application of any applied stress.